In nuclear radiation measurement techniques and also in photometry use is made of detectors such as scintillation counters, proportional counter tubes, semiconductors, or photomultipliers having internal amplification, which supply pulses whose pulse amplitude corresponds to the energy absorbed by the detector. A particular difficulty arises by reason of drift in such radiation measurement means, especially because of variations in internal amplification. The latter variation gives rise to a variation in pulse heights and consequently to a displacement of the energy lines or energy peaks with respect to the adjusted response thresholds of pulse registration, and hence to faulty measurements of intensity. These faults can of course be kept within acceptable limits by regular checking with calibrating radiation sources and manual adjustment. However, automatic control methods and devices for stabilisation of drift are preferable.
For the measurement of cosmic radiation, for example, it is known to use a gamma emitter as a calibrating radiation source and, by means of an integral discriminator and a control device, to maintain a constant rate of pulses exceeding the threshold of the integral discriminator. In this case the pulses produced by the cosmic radiation being measured are greater than the pulses of the calibration radiation and the fluctuating rate of the radiation being measured should be sufficiently low in relation to the pulse rate produced by the calibrating radiation source (R. Ascoli, Il Nuovo Cimento, Vol. IX, No. 7 (1952), Pages 615-617. Instead of a gamma emitter it is also known to use as the external reference radiation source an alpha emitter, particularly by doping a scintillation crystal of a photomultiplier with Americium 241 (British Patent Specification No. 1,067,053).
The use of external radiation sources however involves outlay and is expensive, and may also give rise to the super-imposition of reference peaks on the spectrum that is to be measured.
When pulsed light sources are, for example, used as external reference radiation sources, these have the disadvantage of themselves being subject to drift.
In order to avoid such disadvantages it is already known (German laid-open Specification No. 18 09 520) for the radiation source being measured to be used itself as a reference radiation source and for the controlling variable used for the automatic stabilisation of drift to be derived from a peak of the pulse amplitude spectrum. For this to be possible, however, the pulse amplitude spectrum of the radiation measured must, of course, have a peak suitable for the control process.